Heretofore there has been proposed an arbitrary-direction tracer control system which makes it possible to feed a tracer head and a model relative to each other at an arbitrary angle to the X- or Y-axis (Pat. Appln. No. 14098/79). By such setting of the direction of feed at an arbitrary angle to the X- or Y-axis, even four corners of a square model could be traced with high accuracy.
Even with the arbitray-direction tracer control system, however, there are cases where models cannot be traced with high accuracy according to their configurations. For example, assuming that a model M, whose cross-sectional and top plan views are shown in FIGS. 8A and 8B, is traced along a path depicted in FIG. 8B, portions (a) to (b) and (c) to (d) of the model M where the angle between its curved surfaces and the path is small cannot be traced with high accuracy because the tracer head yields small displacement signals in these portions.